1. Field of the Invention
The present invention relates to compensation for recording medium unevenness during printing operations. More specifically, the present invention relates to control over timing of ink droplet ejection to compensate for recording medium unevenness.
2. Description of the Related Art
Recording medium (paper) unevenness is a known phenomena in ink-jet printing operations. The recording medium unevenness (sometimes called xe2x80x9ccocklingxe2x80x9d) is caused by excessive wetting of the paper by the liquid ink. The cockling introduces an unknown waveform shape into the paper that causes problems during printing operations, such as interference with a recording head during scanning. That is, high spots in the waveform shape of the paper interfere or rub against the recording head as it scans across the paper. The interference can cause problems such as clogging of the ink nozzles on the recording head and smearing of the ink.
To minimize interference problems caused by cockling, it has been proposed to apply pressure to the paper ahead of the recording head as it scans across the paper. One way this has been done is to provide a smaller roller on the printer carriage ahead of the print head such that, as the roller scans across the paper, the roller flattens the uneven paper ahead of the print head. However, the roller only slightly reduces the amount of cockling in the paper and after the roller flattens the paper, the paper tends to return to its uneven condition. Therefore, although the roller somewhat reduces the possibility of interference with the recording head, other problems associated with paper cockling still exist.
Another problem associated with paper cockling is image roughness that is caused by an uneven spacing of the ink droplets as they contact the paper. The ink droplet spacing is dependent upon several factors, including the carriage speed, the ink ejection speed and the distance between the print head and the paper. As seen in FIGS. 13A and 13B, ink droplets are ejected by the recording head at a constant frequency (f) along the scan direction. If the paper is flat or at least very close to being flat as seen in FIG. 13A, the ink droplets contact the paper at approximately the same spacing (d). However, when cockling occurs in the paper and the paper takes on a waveform shape as seen in FIG. 13B, the ink droplets do not contact the paper with a constant spacing, but rather they contact the paper with a different and varying spacing. That is, although the ink droplets are ejected by the recording head at a constant frequency f, the waveform shape of the paper causes some of the ink droplets to contact the paper in a more narrow pattern (d1) than they were ejected at, and some of the ink droplets to contact the paper in a wider pattern (d2) than they were ejected at. Thus, the waveform shape effects the contact frequency because of the varying distance between the print head and the paper. As a result, even though the ink droplets were ejected at a constant frequency, the spacing between the ink droplets contacting the paper is not the same as the spacing frequency that they were ejected at and image roughness occurs.
This problem is made worse in bi-directional printing modes. In bi-directional printing, a line of ink droplets is printed in a forward scan of the recording head, the paper is advanced one line and then another line of ink droplets is printed in a reverse scan of the recording head. Therefore, in bi-directional scanning, the ink droplet frequency contacting the recording medium varies from line to line, which makes the image roughness even worse than unidirectional scanning.
The present invention addresses the foregoing by inducing a predetermined unevenness pattern into the recording medium and adjusting the frequency of ink droplet ejection based on the induced pattern. As a result, a known unevenness pattern is induced into the recording medium and the ink ejection frequency can be adjusted to compensate for the known unevenness pattern. Therefore, ink droplets contact the recording medium in a more even spacing along a scan direction and image density roughness that would otherwise occur is reduced.
Accordingly, in one aspect the invention may be inducing a predetermined unevenness pattern into a recording medium, adjusting an ink ejection frequency of a print head based on the induced unevenness pattern, and controlling ink ejection based on the adjusted frequency.
The invention may be implemented with multiple print heads and in bi-directional printing. Each print head can be controlled by the same control signal, or individually based on the color of ink that the print head ejects, as well as based on whether the print head is scanning in a forward or reverse direction.
Each print head can be controlled with the same control signal, especially if the print heads are spaced relative to one another a distance corresponding to the spacing between the cockling ribs. Spacing the print heads relative to one another a distance corresponding to the distance between the cockling ribs allows both color and black print data can be compensated for accordingly with the same control signal. However, if the print heads are not spaced relative to one another a distance corresponding the distance between the cockling ribs, then if color and black data are to be printed, the color print head may be controlled, and if only black data is to be printed, the black print head can be controlled. Additionally, bi-directional compensation can be provided for, thereby resulting in less density unevenness of mixed color images as well as bi-directional printed images.
The invention may further provide for automatically setting parameters utilized in adjusting the ink ejection frequency based on a selected printing mode. The parameters used in adjusting the frequency can be set based on, for example, whether a single pass or a multi-pass printing mode is selected, the paper type selected (i.e. plain paper, high resolution paper, etc.), and the ink density (i.e. darkness and brightness).
In another aspect, the invention may be adjusting an ink ejection frequency in an ink-jet printer by providing a predetermined recording unevenness pattern having periodic oscillations, dividing each periodic oscillation into a predetermined number of regions, setting a number of ink droplets to be ejected within each region, determining a gate array interval for each region, providing at least one parameter corresponding to a printing operation, and determining an ink ejection trigger difference for each ink droplet to be ejected within each region.